SUN'S RAYS 



85 



with Indian ink to imitate the colour of a negro's skin, and then 

 exposed it for three hours to a very hot sun. For a time the skin 

 remained normal, showing only a little redness at the edge of the 

 black paint, but in due course it became red and inflamed, except 

 where it had been painted black. After several days the erythema 

 disappeared, and the area which had been red was noticed to have 

 become distinctly pigmented, or, in plain language, was sunburnt. 



He then exposed it again to the sun, but this time the area which 

 had originally been covered with Indian ink was unprotected. 

 The result was that this part became red and inflamed, while the 

 pigmented area was unaffected, except that it became a little more 

 pigmented. 



This experiment of Finsen's shows that the colour of native races 

 living in the tropics is protective, and was possibly originally induced 

 by the sun. Sambon and Baly found that the ultra-violet rays were 

 entirely absorbed by a pigmented piece of skin, and therefore support 

 the theory that pigmentation affords an efficient natural protection 

 against ultra-violet rays. 



Dyson studied cutaneous pigmentation in 191 1, and concluded that the 

 formation of melanotic pigment was a normal function of the nuclei of the 

 epidermal cell, metabolism being due to a lipochrome, in which melanin is 

 the chromatic protein portion. This pigment passes to the cutis by way of 

 the lymph stream. The formation of the pigment can be increased by agents 

 such as light, heat, and toxins, but the increase is usually transitory and tends 

 to disappear; but if there is deficient drainage of the fymph from the cutis 

 the pigment accumulates, and unless the drainage is improved tends to become 

 progressively worse. Decreased vitality of the cells may possibly cause the 

 overproduction of pigment due to diminution in nutrition, but if the cells 

 become functionally inactive then pigment production ceases, and the existing 

 pigment being carried away by the blood-stream, the part becomes depig- 

 mented, which is probably the case in vitiligo, in which, after hyperpigmenta- 

 tion, the skin becomes depigmented. 



That the dark pigmentation is useful is shown also by the observa- 

 tion made by us on various occasions that natives suffering from 

 leucoderma avoid going into the sun, as they state that the unpig- 

 mented portions of the skin become inflamed and painful. In this 

 connection, an interesting case showing the importance of these rays 

 came under our notice. A native, who had developed large leuco- 

 dermic patches involving the whole of the face, noticed that 

 he could no longer work in the sun, as each time he tried to do so the 

 patches became painful and he felt sick, very weak, and giddy, and 

 therefore asked to be relieved of his outdoor duties; but it was held 

 that he was still fit to discharge his ordinary duties, with the result 

 that the unfortunate man was compelled to resign his appointment 

 a few months later. It is therefore well that the importance of 

 skin pigmentation should be more widely known. 



Generally speaking, in regions where the sunlight is very intense the race 

 is densely pigmented. As a rule, the people who live nearer the Equator 

 are more pigmented than those farther away — e.g., the black colour of the 

 West African can be compared with the red of the American Indian or the 

 yellow of the Chinese. But this rule is not without exceptions, for the 



